Horizontal drilling technology has come a long way in the past 20 years, and is now an accepted drilling method that has numerous benefits for the recovery of hydrocarbons. Horizontal drilling can be used as both an exploration tool and as a completion technique. The benefits of horizontal drilling when used as part of a completion method include increased drainage area, connecting fracture permeability to the well bore, and reducing drawdown pressures. There also is a strong desire in the industry to reduce the surface foot print caused by drilling activities, and horizontal drilling has proven to be an effective means of reducing the number of wells required to develop a field.
Horizontal drilling is critical for exploiting reservoirs that have little to no primary permeability. To achieve maximum productivity, a horizontal well can be oriented in a particular direction to maximize the number of fractures that the well intersects. By connecting fractures to a well bore, horizontal drilling has been able to turn economically unproductive reservoirs into economic successes. Vertical wells have a much lower probability than horizontal wells of repeatedly intersecting fractures, because nearly all fractures are vertically oriented. A properly placed horizontal well also has been shown to dramatically lower the drawdown pressure across the face of the well bore, and, thus, horizontal drilling also can be applied to water drive reservoirs to eliminate coning.
Generally, a horizontal well comprises at least three distinct segments. First, a vertical borehole extends from the surface to a desired depth beneath the surface, at which point a second, non-linear (i.e. “curved”) borehole transitions the vertical borehole to a third borehole segment (i.e. the “horizontal” segment). The orientation of the third borehole segment, though, depends upon the curvature of the second segment. Thus, the third segment is not necessarily horizontal. In principle, the curvature of the second segment can be adjusted to drill a hole to any desired subsurface location or strata. In practice, though, steering a drill bit with sufficient precision to create the desired curvature has proven difficult.
Typical motor-driven, bottom-hole assemblies have a bent housing that tilts the axis of the drill bit to drill a curved borehole. The orientation of the obtuse angle created by the fixed bend is known as “tool face.” The rigid bend in the drill string points the face of the drill bit in a direction that is tangential to the longitudinal axis of the drill string. But because the bent housing is a fixed part of the drill string, a curved hole can be drilled with these conventional devices only when the drill string is not rotating. Consequently, the technique that uses this type of device is commonly referred to as “slide drilling.”
U.S. Pat. No. 5,941,321 (issued Aug. 24, 1999) describes a “rotary steerable” drilling tool that overcomes some of the disadvantages associated with the conventional slide drilling tools, and permits significantly faster penetration rates because of better hole cleaning. The rotary steerable tool is an apparatus for drilling curved boreholes, particularly in applications that require short radius curvatures, commonly referred to in the art as an “aggressive build rate.” The rotary steerable tool of the '321 patent includes a sliding tube mounted for sliding movement within the central bore of the drill pipe sub-assembly. The upper end of the sliding tube is provided with a tapered throat that makes the sliding tube responsive to pressure from fluid flowing through the drill string. Fluid pressure pushes a deflection device against the side of the borehole, urging the lower end of the drill string to be tilted away from the longitudinal axis of the borehole above the drill bit such that the drill bit will tend to drill in a direction away from the longitudinal axis of the borehole. A knuckle joint also can be included in the drill string between the rotary steerable tool and the drill bit, which can decrease the radius of curvature of a non-linear borehole.
While the rotary steerable tool disclosed in the '321 patent overcomes many disadvantages of the conventional slide drilling procedures, there still remains room for improvement. In particular, the tapered throat on the upper end of the sliding tube restricts the flow of drilling fluid as it passes through the drill string. Such a fluid restriction can increase the pressure above the tool and adversely affect the bit hydraulics, requiring more powerful and more expensive fluid pumps to compensate for the restriction. Additionally, the rotation of the drill pipe tends to cause the eccentric sleeve of the tool to rotate within the borehole, which can cause the deflection device to collapse or steer the drill bit in an undesired direction.